An organic electroluminescent device (hereinafter, “electroluminescent” is often abbreviated as “EL”) is a self-emission device by the use of the principle that a fluorescent compound emits light by the recombination energy of holes injected from an anode and electrons injected from a cathode when an electric field is impressed.
Since C. W. Tang et al. of Eastman Kodak Co. reported a low-voltage driven organic EL device in the form of a stacked type device (Non-Patent Document 1), studies on organic EL devices wherein organic materials are used as the constituent materials has actively conducted.
In the organic EL device reported by Tang et al., tris(8-hydroxyquinolinol)aluminum is used for an emitting layer, and a triphenyldiamine derivative is used for a hole-transporting layer. The advantages of the stack structure are to increase injection efficiency of holes to the emitting layer, to increase generation efficiency of excitons generated by recombination by blocking electrons injected in the cathode, to confine the generated excitons in the emitting layer, and so on.
Like this example, as the structure of the organic EL device, a two-layered type of a hole-transporting (injecting) layer and an electron-transporting emitting layer, and a three-layered type of a hole-transporting (injecting) layer, an emitting layer and an electron-transporting (injecting) layer are widely known. In such stack structure devices, their device structures and fabrication methods have been contrived to increase recombination efficiency of injected holes and electrons.
As the hole-injecting material used in an organic EL device, a material having a phenylenediamine structure is known from Patent Documents 1 and 2 and has heretofore been used widely. As the hole-transporting material, an arylamine material containing a bendizine structure described in Patent Documents 3 and 4 has been used.
Patent Documents 5 to 7 disclose a carbazole-containing arylamine compound. The compound increases luminous efficiency when used as a hole-transporting material, but has such a disadvantage that the driving voltage significantly increases which results in an extremely shortened device life.
For effective injection of holes from an anode to an emitting layer, Patent Document 8 discloses a device having two or more hole-injecting/transporting layers having ionization potential values which are set in a stepwise manner. The material described in Patent Document 8 is not satisfactory in respect both of luminous efficiency and device life.
Patent Document 1: JP-A-H8-291115
Patent Document 2: JP-A-2000-309566
Patent Document 3: U.S. Pat. No. 5,061,569
Patent Document 4: JP-A-2001-273978
Patent Document 5: U.S. Pat. No. 6,242,115
Patent Document 6: JP-A-2000-302756
Patent Document 7: JP-A-H11-144873
Patent Document 8: JP-A-H6-314594
Non-patent Document 1: C. W. Tang, S. A. Vanslyke, Applied Physics Letters, 51, 913, 1987
An object of the invention is to provide a low-voltage, high-efficiency, and long-lived organic EL device.